1. Field of the Invention
The present invention relates to a connecting rod and a production method thereof. Moreover, the present invention also relates to an internal combustion engine and an automotive vehicle including such a connecting rod.
2. Description of the Related Art
In an internal combustion engine of an automotive vehicle, a part which is called a connecting rod (or con rod) is used for linking a crankshaft to a piston. FIG. 16 shows a conventional connecting rod 401. The connecting rod 401 includes a bar-like rod main body 410, a small end 420 which is provided at one end of the rod main body 410, and a big end 430 which is provided at the other end of the rod main body 410.
The small end 420, which has a piston pin hole 425 for allowing a piston pin to extend therethrough, is connected to a piston via the piston pin. On the other hand, the big end 430, which has a crankpin hole 435 for allowing a crankpin to extend therethrough, is connected to a crankshaft via the crankpin.
The big end 430 is split into a rod portion 433 which continues from one end of the rod main body 410, and a cap portion 434 which is coupled to the rod portion 433 with bolts 440. Therefore, the connecting rod 401 shown in FIG. 16 is referred to as a split-type connecting rod. Other types of connecting rods are also known whose big end is not split.
Conventionally, steel has widely been used as a material for connecting rods. Recently, use of a titanium alloy has been disclosed (see, for example, Japanese Laid-Open Patent Publication No. 60-247432). Using a titanium alloy to reduce the weight of a connecting rod allows the weight of the crankshaft and other engine parts to be also reduced, whereby the weight of the entire engine can be further reduced. This makes possible an improved output power and reduced fuel consumption.
However, it has been found that using a titanium alloy as the material of a connecting rod may result in the following problems.
Requirements on the mechanical characteristics of a connecting rod differ from region to region. For example, the small end and rod main body of a connecting rod are required to have a high strength and a high toughness so as not to undergo fatigue failure or impact failure during use. On the other hand, the big end of a connecting rod is required to be such that the inner peripheral surface of the crankpin hole does not deform due to a rotary motion of a crankshaft, so that there is little friction against a crankpin. That is, the big end must have a high rigidity and a high elastic modulus so as not to deform during a high-speed drive.
If a titanium alloy whose elastic modulus is lower than that of steel is used as the material of a connecting rod, weight reduction may be achieved, but the rigidity of the big end will be insufficient. On the other hand, designing a connection rod to have a size for attaining sufficient rigidity will detract from the advantage of weight reduction.
Accordingly, the inventors have contemplated using different metal materials for different regions of a connecting rod. In a connecting rod which is formed by bonding together a plurality of members which are composed of metals of different compositions, desired mechanical characteristics can be conferred to different regions of the connecting rod. For example, a metal material which is light-weight and which has a good fatigue strength may used for the small end and the rod main body, whereas a metal material which has a high rigidity and a high elastic modulus may be used for the big end, whereby weight reduction can be achieved while ensuring rigidity of the big end.
However, the inventors have conducted detailed studies to find that adequate weight reduction may actually be difficult to achieve even with such a technique. A connecting rod transmits the explosive force from a piston to a crankshaft, and therefore is required to have a high strength. Hence, if a connecting rod is formed by bonding together a plurality of members, the joints will have a reduced strength, thus making it difficult to obtain a sufficient strength. On the other hand, if a thick connecting rod is built in order to increase the cross-sectional area at each such joint and compensate for the reduced strength, there will be a corresponding increase in its weight, which detracts from the purpose of using different metal materials.